/*
 * Copyright (C) 2007 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.collect;

import com.google.common.annotations.GwtCompatible;
import java.io.Serializable;

/**
 * An abstract base class for implementing the
 * <a href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>. The
 * {@link #delegate()} method must be overridden to return the instance being decorated.
 *
 * <p>
 * This class does <i>not</i> forward the {@code hashCode} and {@code equals} methods through to the
 * backing object, but relies on {@code Object}'s implementation. This is necessary to preserve the
 * symmetry of {@code equals}. Custom definitions of equality are usually based on an interface,
 * such as {@code Set} or {@code List}, so that the implementation of {@code equals} can cast the
 * object being tested for equality to the custom interface. {@code
 * ForwardingObject} implements no such custom interfaces directly; they are implemented only in
 * subclasses. Therefore, forwarding {@code equals} would break symmetry, as the forwarding object
 * might consider itself equal to the object being tested, but the reverse could not be true. This
 * behavior is consistent with the JDK's collection wrappers, such as
 * {@link java.util.Collections#unmodifiableCollection}. Use an interface-specific subclass of
 * {@code ForwardingObject}, such as {@link ForwardingList}, to preserve equality behavior, or
 * override {@code equals} directly.
 *
 * <p>
 * The {@code toString} method is forwarded to the delegate. Although this class does not implement
 * {@link Serializable}, a serializable subclass may be created since this class has a
 * parameter-less constructor.
 *
 * @author Mike Bostock
 * @since 2.0
 */
@GwtCompatible
public abstract class ForwardingObject {

    /** Constructor for use by subclasses. */
    protected ForwardingObject() {}

    /**
     * Returns the backing delegate instance that methods are forwarded to. Abstract subclasses
     * generally override this method with an abstract method that has a more specific return type,
     * such as {@link ForwardingSet#delegate}. Concrete subclasses override this method to supply
     * the instance being decorated.
     */
    protected abstract Object delegate();

    /**
     * Returns the string representation generated by the delegate's {@code toString} method.
     */
    @Override
    public String toString() {
        return delegate().toString();
    }

    /* No equals or hashCode. See class comments for details. */
}
